Introduction: epidote is frequently observed in low to medium-grade metamorphic mineral assemblages, particularly in basic igneous rocks. Likewise, it occurs in metamorphism-related mineral veins cutting such rocks. Hydrothermal alteration may also include the development of epidote and finally it is frequently observed in skarn-type associations developed during contact metamorphism of calcareous rocks. Care must be taken when examining material carrying epidote-like minerals because epidote forms a solid solution series with clinozoisite. However, in many Welsh occurrences of these minerals, colour has proved to be a reasonable guide: epidote is a characteristic pistachio-green colour while clinozoisite is greenish through to brown.

Occurrence in Wales: the presence of epidote in Wales was first mentioned by Greenly (1919), while more recently, electron microprobe analyses were provided by Bevins & Rowbotham (1983) and by Bevins & Merriman (1988). Epidote is a widespread accessory mineral in Lower Palaeozoic low-grade metabasites outcropping in many parts of Wales, particularly in the north and west where the majority of them outcrop. A good example of hydrothermally-added epidote (so-called epidotization) is within the Rhobell Volcanics of early Ordovician age, outcropping to the NE of Dolgellau in Gwynedd: the rocks themselves have a striking bright greenish colour due mostly to the abundant presence of epidote. The localities listed below are where well-crystallized epidote has been recorded.

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